Fluorescent inks and toners may be used as an authenticating feature in the document security industry. Secure documents, for example documents that are difficult to forge, may be created using inks or toners that include fluorescent agents either alone or in combination with ordinary inks and/or pigments. Features printed using fluorescent inks or toners are usually invisible under visible light, due to the colorless nature of the security inks or due to masking by other colorants in the document. Under ultraviolet illumination, however, the fluorescent features of the document are revealed in the form of a bright emission by the fluorescent dyes in the visible spectrum. For example, certain bank notes utilize visible features, such as holographic patches, microprinting and microtextures to conceal additional fluorescent threads and/or multi-colored emblems embedded in the bank note, which are only revealed under specific light frequencies. These features provide an increased level of security against counterfeiters by making the copying process of such a document more difficult.
The term “fluorescent dye” refers to a fluorescent material that is soluble like any other organic molecule in a vehicle and easily makes homogeneous printing compositions.
The term “fluorescent pigment” refers to a fluorescent material that is insoluble in a vehicle and generally requires substituted uniform dispersion in the vehicle to use it. In most cases, the only medium available that may dissolve fluorescent pigments is a strong acid, such as concentrated sulfuric acid.
Fluorescent dyes have typically been used for fluorescent inks for xerographic and electrographic printing of security features. However, a major drawback of fluorescent dyes is that they degrade thermally. For example, the fluorescence can be lost after about 12 days of continuous heating at 125° C. This drawback is detrimental with respect to solid ink printers because the printers need to be powered, requiring high temperature, for an extended time, which has an adverse effect on the fluorescent dye.
Generally, pigments are considered the better alternative because of their improved chemical, light fastness and thermal stability. They are also preferred by the industry because there is limited or no migration or bleeding of the colorant compound, which more easily occurs with dyes. Pigments may also be significantly less expensive than dyes, and so are attractive colorants for use in printing inks.
To overcome the problems associated with fluorescent dyes described above, the security printing industry uses hard, robust pigments containing the dye of interest. These daylight fluorescent pigments are made of a hard cross-linked polymer matrix incorporating fluorescent dyes, and are dispersed in the marking vehicle, typically liquid inks. In the hard pigment particle, the dye is isolated from interaction with other materials present in the ink and as a result, chemical degradation by the environment is prevented. In addition, mobility of the dye is severely restricted by the hard polymer matrix, which is required for any thermal degradation process.
However, these hard pigment particles also present drawbacks. For example, the size of commercially available daylight fluorescent pigments is about 3-5 microns and even higher. Currently, inks based on fluorescent pigments are being printed by rotogravure, flexographic, silk-screening and off-set systems. Given their large size, these pigments cannot be used in ink jet printing, such as ink jet printing using solid inks or UV curable inks, because the pigments would produce physical clogging of the ink jet nozzles. For example, when preparing solid ink compositions by adding a conventional pigment to a solid ink base, pigments having a size of about 1 micron is cannot be used because of their tendency to plug the nozzles of the ink jet printer, due to their large size.